1. Field of the Invention
The present invention relates to an elastic wave duplexer and more particularly to an elastic wave duplexer, which is flip-chip mounted to a substrate and which includes a transmission elastic wave filter element and a reception elastic wave filter element, both the filter elements being sealed off.
2. Description of the Related Art
Various conventional surface acoustic (elastic) wave devices have been proposed in which surface acoustic wave elements are mounted to a substrate and are sealed off by using a sealing resin.
For example, Japanese Unexamined Patent Application Publication No. 2002-100945 discloses a surface acoustic wave device illustrated in a sectional view of FIG. 15. The disclosed surface acoustic wave device includes, as illustrated in FIG. 15, a mounting substrate 110, a surface acoustic wave element 120 mounted onto the mounting substrate 110, and a sealing member 130 for air-tightly sealing off the surface acoustic wave element 120. After forming a plurality of surface acoustic wave devices on a bulk substrate 140, the surface acoustic wave devices are cut at cut positions 141 into individual devices. The mounting substrate 110 includes conductor patterns 111 formed on one surface thereof. The conductor patterns 111 are extended to pass through the mounting substrate 110 and are connected to electrodes disposed on the other surface of the mounting substrate 110. The mounting substrate 110 is made of, e.g., a ceramic or a resin. The surface elastic wave element 120 includes a piezoelectric substrate 121, comb-shaped electrodes 122 and conductor patterns 123 both formed on one surface of the piezoelectric substrate 121, and bumps 124 made of, e.g., gold and formed at ends of the conductor patterns 123. The conductor patterns 123 are electrically connected to the comb-shaped electrodes 122. The surface acoustic wave element 120 is an element utilizing surface acoustic waves generated by the comb-shaped electrodes 122, and it is employed as a filter element, a resonator, etc.
The surface acoustic wave element 120 is mounted onto the mounting substrate 110 such that the comb-shaped electrodes 122 and the one surface of the mounting substrate 110 are positioned to face each other while a space 133 is formed therebetween. Further, the bumps (connecting electrodes) 124 are electrically connected to the conductor patterns 111 on the mounting substrate 110 by flip-chip bonding. The sealing member 130 is made of a sealing material 150 applied so as to cover the surface acoustic wave element 120 except for the space 133 that is formed between the comb-shaped electrodes 122 and the one surface of the mounting substrate 110. The sealing material 150 is, e.g., a resin which has, before a hardening process, not only fluidity, but also viscosity at such a level as to not allow the resin to easily enter the space 133, and which is hardened and dried by the hardening process.
Also, Japanese Unexamined Patent Application Publication No. 2003-87095 discloses a surface acoustic wave device illustrated in FIGS. 16A and 16B. FIG. 16A is a perspective view of the surface acoustic wave device. FIG. 16B is a sectional view taken along a line A-A′ in FIG. 16A. The disclosed surface acoustic wave device includes, as illustrated in FIGS. 16A and 16B, a surface acoustic wave element 201 having comb-shaped electrodes 214a and 214b, bumps 205a to 205g disposed on the surface acoustic wave element 201, a base substrate 204 electrically and mechanically connected to the surface acoustic wave element 201 through the bumps 205a to 205g, and a sealing member 202 for protecting the surface acoustic wave element 201 against mechanical stresses and environmental stresses. The surface acoustic wave element 201 is mounted to the base substrate 204 by applying ultrasonic waves to the surface acoustic wave element 201 to melt the bumps 205a to 205g, and by joining the base substrate 204 and the surface acoustic wave element 201 to each other. A surface of the surface acoustic wave element 201 on which the comb-shaped electrodes 214a and 214b are disposed is sealed off by the sealing member 202 that is coated over the base substrate 204 and the backside of the surface acoustic wave element 201. The bumps 205a to 205g are made of, e.g., gold or silver. The sealing member 202 functions as a surface protective film for the surface acoustic wave element 201. Thus, the sealing member 202 can protect the surface acoustic wave element 201 against mechanical stresses and environmental stresses. The sealing member 202 is made of, e.g., a polymeric material, such as a polyimide resin or a PP/EPR-based polymer alloy. The surface acoustic wave element 201 includes a piezoelectric substrate 203 made of, e.g., lithium tantalate or lithium niobate, the comb-shaped electrodes 214a and 214b disposed on one principal surface of the piezoelectric substrate 203, which is positioned to face the base substrate 204, and bonding pads electrically connected to the comb-shaped electrodes 214a and 214b and disposed on the same plane on which the comb-shaped electrodes 214a and 214b are disposed. In addition, the bumps 205a to 205g are connected to the bonding pads for supplying signals, etc., to the comb-shaped electrodes 214a and 214b from the base substrate 204.
When the structures illustrated in FIG. 15 and FIGS. 16A and 16B are each applied to an elastic wave duplexer including a surface acoustic wave filter element for transmission and a surface acoustic wave filter element for reception, the surface acoustic wave filter element for transmission and the surface acoustic wave filter element for reception are flip-chip mounted to a substrate and are sealed off by using a sealing resin. One principal surface of the surface acoustic wave filter element, on which comb-shaped electrodes are formed, is positioned to face the substrate, and a sealing member made of a polymeric material (resin) is coated over the other principal surface of the surface acoustic wave filter element on the side farthest away from the substrate. Direct reaching waves are generated due to capacitance that is produced on the other principal surface of the surface acoustic wave filter element with the presence of the sealing member.
More specifically, assuming, for example, a surface acoustic wave filter element 60 having two terminal pairs, i.e., input terminals 61 and 62 and output terminals 63 and 64, as illustrated in a circuit diagram of FIG. 11, there occur signals that are directly transmitted from the input terminals 61 and 62 to the output terminals 63 and 64 without passing through the surface acoustic wave filter element 60 as indicated by arrows 70 and 72 in a circuit diagram of FIG. 12. Those signals are called “direct reaching waves”.
As illustrated in an equivalent circuit diagram of FIG. 13, the direct reaching waves include a component 74 attributable to mutual inductance between the input terminals 61 and 62 and the output terminals 63 and 64, a component 76 attributable to capacitive coupling between the input terminals 61 and 62 and the output terminals 63 and 64, a component 78 attributable to floating of the ground, etc.
The direct reaching waves degrade an isolation characteristic between a transmission terminal and a first reception terminal and between the transmission terminal and a second reception terminal of the elastic wave duplexer.